Ultrasound imaging system

ABSTRACT

The disclosure is directed to an apparatus for scanning an object with a beam of ultrasound energy and for formulating image-representative signals from the ultrasound reflected from the object. An ultrasound reflector is disposed in the path of the ultrasound energy, the reflector typically being disposed in a suitable fluid, for example water. The reflector is mechanically driven and means are provided for sensing the relative angular position of the reflector and for generating a first clock signal as a function of the sensed position. Means, responsive to the ultrasound reflected from the object are provided for generating echo-representative electrical signals. These echo-representative electrical signals are stored at a line rate which depends upon the first clock signal. Finally, means are provided for reading out the stored signals at a line rate which depends upon a second clock signal, and for displaying the read out signals to obtain an image of the object, the line rate of the display being synchronized with the second clock rate.

This is a continuation-in-part of U.S. application Ser. No. 806,004, nowabandoned filed June 13, 1977.

BACKGROUND OF THE INVENTION

This invention relates to ultrasonic systems and, more particularly, toapparatus for imaging sections of a body by transmitting ultrasonicenergy into the body and determining the characteristics of theultrasonic energy reflected therefrom.

During the past two decades ultrasonic techniques have become moreprevalent in clinical diagnosis. Such techniques have been utilized forsome time in the field of obstetrics, neurology and cardiology, and arebecoming increasingly important in the visualization of subcutaneousblood vessels including imaging of smaller blood vessels.

Various fundamental factors have given rise to the increased use ofultrasonic techniques. Ultrasound differs from other forms of radiationin its interaction with living systems in that it has the nature of amechanical wave. Accordingly, information is available from its usewhich is of a different nature than that obtained by other methods andit is found to be complementary to other diagnostic methods, such asthose employing X-rays. Also, the risk of tissue damage using ultrasoundappears to be much less than the apparant risk associated with ionizingradiations such as X-rays.

The majority of diagnostic techniques using ultrasound are based on thepulse-echo method wherein pulses of ultrasonic energy are periodicallygenerated by a suitable piezoelectric transducer such as a leadzirconate-titanate ceramic. Each short pulse of ultrasonic energy isfocused to a narrow beam which is transmitted into the patient's bodywherein it eventually encounters interfaces between various differentstructures of the body. When there is a characteristic impedancemismatch at an interface, a portion of the ultrasonic energy isreflected at the boundary back toward the transducer. After generationof the pulse, the transducer operates in a "listening" mode wherein itconverts received reflected energy or "echoes" from the body back intoelectrical signals. The time of arrival of these echoes depends on theranges of the interfaces encountered and the propagation velocity of theultrasound. Also, the amplitude of the echo is indicative of thereflection properties of the interface and, accordingly, of the natureof the characteristic structures forming the interface.

There are various ways in which the information in the received echoescan be usefully presented. In one common technique, the electricalsignal representative of detected echoes are amplified and applied tothe vertical deflection plates of a cathode ray display. The output of atime-base generator is applied to the horizontal deflection plates.Continuous repetition of the pulse/echo process in synchronism with thetime-base signals produces a continuous display, called an "A-scan", inwhich time is proportional to range, and deflections in the verticaldirection represent the presence of interfaces. The height of thesevertical deflections is representative of echo strength.

Another common form of display is the so-called "B-scan" wherein theecho information is of a form more similar to conventional televisiondisplay; i.e., the received echo signals are utilized to modulate thebrightness of the display at each point scanned. This type of display isfound especially useful when the ultrasonic energy is scanned transversethe body so that individual "ranging" information yields individualscanlines on the display, and successive transverse positions areutilized to obtain successive scanlines on the display. This type oftechnique yields a cross-sectional picture in the plane of the scan, andthe resultant display can be viewed directly or recordedphotographically or on magnetic tape. The transverse scan of the beammay be achieved by a reflector which is scanned mechanically over adesired angle. It is generally considered desirable that the ultrasoundreflector be mechanically scanned at a linear rate which is compatiblewith a given field rate of a television-type display. Accordingly, anacceptable scan pattern would be controlled by a sawtooth energizingwaveform which rises linearly over most of its period and has arelatively short "flyback" (related to vertical blanking of thedisplay), the flyback being as short as possible so that the operationalduty cycle is as high as possible.

The described scan of the ultrasound reflector is consideredadvantageous from the standpoint of compatibility with television-typedisplays and is intended to provide advantageous linearity during thescan, but it is found that operational difficulties arise. Inparticular, it is known that ultrasound is highly reflected atliquid-gas interfaces and this has led to the technique of coupling theultrasound through a fluid. When the reflective scanner is contained ina fluid medium such as water, linear movement and relatively fastflyback can be difficult to attain and/or be mechanically inefficientrequiring high input power. This is especially true for equipmentswherein relatively large-area scanners, with their attendant inertialresistance to acceleration and deceleration in a fluid, are utilized.

It is one of the objects of this invention to provide an imaging systemwhich is generally responsive to the problems of the prior art, as setforth.

SUMMARY OF THE INVENTION

The present invention is directed to an apparatus for scanning an objectwith a beam of ultrasound energy and for formulatingimage-representative signals from the ultrasound reflected from theobject. In accordance with the invention, an ultrasound reflector isdisposed in the path of the ultrasound energy, the reflector typicallybeing disposed in a suitable fluid, for example water. The reflector ismechanically driven and means are provided for sensing the relativeangular position of the reflector and for generating a first clocksignal as a function of the sensed position. Means, responsive to theultrasound reflected from the object, are provided for generatingecho-representative electrical signals. These echo-representativeelectrical signals are stored at a line rate which depends upon thefirst clock signal. Finally, means are provided for reading out thestored signals at a line rate which depends upon a second clock signal,and for displaying the read out signals to obtain an image of theobject, the line rate of the display being synchronized with the secondclock rate.

In the preferred embodiment of the invention, the means for driving thereflector is adapted to drive the reflector at a non-linear rate, forexample, sinusoidally. This is advantageous in that the drive can beapplied at a frequency which approximates a resonance of the reflectorin the fluid, and the abrupt motions of a sawtooth drive are avoided. Inthus embodiment, the duty cycle of the scan is enhanced by storing anddisplaying information obtained during both the positive andnegative-going cycles of the sinusoidal drive. In other words,information is obtained and displayed while scanning in both directions,so there is no wasted flyback period. In the preferred embodiment, theelectrical signals are stored in shift registers. The information storedduring an angular sweep of the scanner in one direction is read out on a"first-in-first-out" basis, whereas the information stored during thesweep of the scanner in the opposite direction is read out on a"last-in-first-out" basis.

In an embodiment of the invention, the means for sensing the position ofthe reflector comprises a shaft encoder and the shift registers used forstorage are analogue registers of the charge transfer type.

Further features and advantages of the invention will become morereadily apparent from the following detailed description when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the operation of a scanning apparatus which employsthe improvements of the invention.

FIG. 2 is a schematic diagram, partially in block form, of an apparatusin accordance with the embodiment of the invention.

FIG. 3 is a block diagram of the timing circuitry and the reflectordrive and display sweep circuitry of FIG. 2.

FIG. 4 is a block diagram of an embodiment of the frame storagesubsystem of FIG. 2.

FIG. 5, consisting of FIGS. 5A-5F, shows timing diagrams which areuseful in understanding operation of an embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, there is shown an illustration of a scanningapparatus which employs the improvements of the invention. A console 10is provided with a display 11 which may typically be a cathode ray tubetelevision-type display, and a suitable control panel. A video taperecorder or suitable photographic means may also be included in theconsole to effect ultimate display of images. The console will typicallyhouse power supplies and portions of the timing and processing circuitryof the system to be described. A portable scanning module or probe 50 iscoupled to the console by a cable 48. In the present embodiment theprobe is generally cylindrical in shape and has a scanning window 51near one end, the scanning window being defined by protruding flexiblematerial, which may typically be silicone rubber. During operation ofthe apparatus, the probe 50 is hand-held to position the scanning windowover a part of the body to be imaged. For example, in FIG. 1 the probeis positioned such that a cross section of the heart will be obtained.Imaging of other portions of the body is readily attained by moving theprobe to the desired position and orientation, the relative orientationof the scanning window determining the angle of the cross section taken.

Referring to FIG. 2, there is shown a cross-sectional view of a portionof the scanning module or probe 50 along with diagrams of portions ofthe circuitry therein and in console 10 used in conjunction therewith.An enclosure, which may be formed of plastic, consists of a front cover52 which defines a fluid-tight compartment, and a rear cover 53 whichhouses at least a portion of the system electronics. Both covers aregenerally cylindrical in shape and fit, with suitable seals, over acylindrical inner housing 54 having an annular rim 55. The inner housingholds a transducer 80 and a focusing lens 90, which may be in accordancewith the description set forth in the U.S. Pat. No. 3,958,559. Thescanning window 51 is defined by a generally rectangular opening in theside of cover 52, the opening having a slightly protruding lip on whichis mounted a flexible material 56, which may be a silicone rubbermembrane. The volume of the enclosure defined by front cover 52 isfilled with a fluid 57, for example, water. The membrane 56 willaccordingly conform in shape to the surface of a body portion with whichit is placed in contact, thereby minimizing the possibility of anundesirable reflective liquid-air interface.

A reflective scanner 70, which is flat in the present embodiment butwhich may be curved to provide focusing if desired, is disposed in thefluid 57 between the lens 90 and the scanning window 51. The scanner 70is mounted on a shaft 71 (perpendicular to the plane of the paper) whichpasses through a suitable seal in cover 52 and is connected to a smallelectric motor 72 which is mounted on the outside of cover 52 andprovides the desired oscillatory motion. A shaft encoder unit 73, whichis shown schematically in the insert in FIG. 2, is mounted partially onthe shaft and partially on the cover 52. The motor and shaft encoder,which are shown in dashed line in the FIGURE, may be provided with asmall separate cover (not shown) which constitutes a protrusion on thecover 52, or an irregular outer shape can be avoided by providing asecondary larger outer shell (not shown).

The transducer 80 is coupled to a pulser/receiver circuit 130 whichalternately provides energizing pulses to and receives echo signals fromthe transducer 80. If desired, coupling between the pulser/receiver 130and the transducer 80 may be via variable delay circuitry (not shown)which includes variable delay elements and provides dynamic focusing ofthe ultrasound beam, in a manner well known in the art. Dynamic focusingcircuitry is generally employed in conjunction with a segmentedtransducer and, if desired, a stepped focusing lens, as disclosed in thecopending U.S. application Ser. No. 665,898, now U.S. Pat. No.4,084,582, assigned to the same assignee as the present application. Thereceiver portion of circuit 130 includes a preamplifier and is coupledto a frame storage subsystem 200 (via line 130A). The output of theframe storage subsystem 200 is coupled to display 11 and recorder 160,which may be any suitable recording or memory means such as a video taperecorder. If desired, gain control circuitry may be provided and mayinclude interactive gain compensation, which is described in detail inU.S. Pat. No. 4,043,181, assigned to the same assignee as the presentapplication. Interactive gain compensation circuitry compensates theamplitude of later arriving signals for attenuation experienced duringpassage through body tissue and losses due to prior reflections. Timingcircuitry 170 generates timing signals which synchronize operation ofthe system; the timing signals being coupled to the circuitry 200 andalso to reflector drive and display sweep circuitry 180 which generatesthe signals that control the oscillation of scanner 70 and the verticaland horizontal sweep signals for the display 11 and recorder 160.

In broad terms, operation of the system is as follows: The pulser incircuitry 130 generates pulses which excite the transducer 80. Theresultant ultrasound energy is focused by the lens 90 and reflected offthe surface of scanner 70 and into the body 5, as represented in FIG. 2,the dashed line depicting the beam outline. When the beam has beentransmitted toward the body, the timing circuitry causes thepulser/receiver 130 to switch into a "receive" or "listen" mode. Now,the transducer 80 serves to convert ultrasound energy, which is in theform of echoes reflected from the body and back off the scanner 70, intoelectrical signals. These signals are coupled, via the frame store unit200, to the display 11. For a "B-scan" display, a sweep over a range ofdepths (which naturally results from the transmitted energy reflectingoff different interfaces in the body) corresponds to a horizontalscanline of the display. The second dimension of the desiredcross-sectional image is obtained by a slower mechanical scan of scanner70, the mechanical scanning range being illustrated by the double-headedarrow 7.

Description in further detail will now be set forth. Clock signals C₁obtained from the shaft encoder control the pulser/receiver 130 toproduce pulses at the rate of one for each scanline of the video signalto be ultimately displayed on the display 11. As is well known in theart, each scanline in a B-scan system represents echo information, withthe later arriving echoes representing successively deeper investigationinto the body. The timing circuitry 170 generates clock signals andtiming signals, to be described, which are coupled to reflector driveand display sweep circuitry 180 and the frame storage subsystem 200. Oneof the timing inputs to the circuitry 180 and subsystem 200 is a clocksignal C₂, which is at a frequency that determines the line rate of thedisplay 11. The remaining timing signals are represented in general inFIG. 2 by a separate cable designated 170A.

The reflector drive and display sweep circuitry 180 generates verticaland horizontal sweep signals that are coupled to the display 11 andrecorder 160 and also generates a motor drive signal, on a line 180A,which is coupled to the motor 72 which provides the desired oscillatorymotion of the scanning reflector 70. In the present embodiment, thereflector drive signal is nonlinear, preferably sinusoidal, rather thanthe usual sawtooth drive waveform (which would typically be similar tothe shape of the waveform of the horizontal sweep signal for display11). The output of shaft encoder 73 is a clock signal, designated asclock C₁ which consists of a train of pulses that represent incrementalangular displacements of the reflector 70. This may be achieved, forexample, and as shown in FIG. 2, by providing a stationary light source73B and photodetector 73C which are separated by an apertured ring 73Dthat has equally spaced apertures thereon and is mounted to move withthe motor shaft. Unlike the clock pulses C₂, which are periodic at theline rate, the clock pulses C₁ will have relative time spacings that areirregular and which relate to the scan velocity of scanner 70 duringdifferent portions of its scan. In the present embodiment, the scanneris driven sinusoidally, and the clock pulses C₁ will be closest togetherduring the center of the scan when the reflector is moving fastest andwill be relatively farther apart when the reflector is at the extremesof its oscillatory scan and slows down to change direction. This isshown, for example, in FIG. 5E which is described further hereinbelow.In the present embodiment, the clock pulses C₁, which are substantiallysynchronized in time with the position of reflector 70 and thus thetransverse position of the ultrasound beam, are utilized to clock linesof echo-representative information into the frame storage subsystem 200,which preferably comprises a charge transfer type of register such as acharge couple device ("CCD") frame store. The information issubsequently clocked out using the periodic clock C₂ to clock out linesof information in synchronization with scanlines of the display 11. Inthis manner, the scanner 70 can be driven in an efficient non-linearmanner, for example near a natural resonance, and accurate imaginginformation can still be displayed using conventional display equipment.

Referring to FIG. 3, there is shown in further detail the timingcircuitry 170 and reflector drive and display sweep circuitry 180utilized in an embodiment of the invention wherein imaging informationis obtained during excursions of the scanner 70 in both directions. Inthis embodiment, the effective duty cycle of the equipment is enhancedsince there is no wasted flyback period, and a smooth sinusoidal driveof the reflector can be employed. Briefly, this is achieved by utilizinga pair of field stores in the frame storage subsystem 200 (FIG. 2),these field stores being described in further detail in conjunction withFIG. 4. When one of the field stores is having new information clockedinto it (during an excursion of the reflector 70 in one direction), theother field store is having previously stored information clocked out ofit for display on the display 11. During the subsequent excursion of thereflector in the opposite direction, the reverse situation is effective,so that the field stores alternately store and read out information tothe display. In FIG. 3, F represents the frame rate at which the display11 is to operate (2F being the field rate), L represents the number oflines at which the display is to operate, and E represents the number ofelements per scanline. Since F will also correspond to the frequency ofoscillation of the reflector 70, it can be selected to be at or near thenatural resonant frequency of the scanner in its fluid environment.Typically, F may be 15 Hz. and L and E selected using considerations ofresolution and bandwidth. For example, one might select 250 lines perfield for an interlaced 500 lines per field, and 500 elements per line,so that L would be 250 and E would be 500.

The basic high frequency clock for the system is the clock C₃ whichproduces clock pulses at a frequency of 2·F·L·E. As will be described,the clock C₃ determines the basic sampling rate at which samples arestored by the frame storage subsystem 200. Phase shift circuitry 171produces three phases of the clock C₃, at relative phases of 120° withrespect to each other. A clock C₂ generates clock pulses at the linerate (2·F·L), this being achieved by dividing the frequency of the clockC₃ by E. A clock designated 2F obtains clock pulses at the rate 2F bydividing the frequency of clock C₂ by L to produce clock pulses at thefield rate. Another clock, designated F, divides the field rate clock bytwo to obtain pulses at the frame rate, F.

As previously noted, the clock C₂, which operates at the nominal linerate, is utilized for clocking line information out of the frame storagesubsystem 200, and is also utilized to synchronize the horizontalscanlines of display 11. The last-mentioned function is indicated by thecoupling to horizontal sweep circuitry 181 which is part of thereflector drive and display sweep circuitry 180. The clock pulses at thefield rate 2F are utilized to synchronize the field rate of the display,as is indicated by the coupling to vertical sweep circuitry 182 in thereflector drive and display sweep circuitry 180. The outputs of circuits181 and 182, which may be conventional sweep generators, are thehorizontal and vertical sweep signals which are typically coupled to thedeflection coils of the display 11. The clock which operates at theframe rate F is utilized to synchronize an oscillator 183 that generatesa sinusoidal drive at the frequency F (as shown in FIG. 5A) forenergizing the reflector motor 72. The clock pulses at the frame rate Fare additionally coupled to an inverter 175 so that clock pulsesdesignated F are also available at the frame storage subsystem 200, theclock F being of opposite phase from the clock F.

Referring to FIG. 4, there is shown an embodiment of the frame storagesubsystem 200 of FIG. 2. A pair of field store registers 210 and 260 areprovided. These field stores are preferably of the analog chargetransfer type, such as three-phase charge coupled device (CCD) fieldstores. Field stores of the general type described are known in the artand reference is made, for example, to the field storage techniquesdisclosed in U.S. Pat. No. 3,882,271. The referenced patent describesthe manner in which a video-representative signal can be sampled andstored in a frame storage register by sampling information on each lineto read a full line of information into the field store, and thenshifting the full line of information to the next row of the field storewhereupon the next line of information is sampled and read in. It willbe understood, however, that various types and arrangements of fieldstores, including field stores which are commercially available, can beutilized in conjunction with the present invention.

To illustrate operation of the field store 210, a line of information isclocked into a row of CCD elements designated row 1, under control ofthe three phases of the clock C₃, in a manner well known in the art. Atthe end of the line, the line of information in row 1 is clocked (inparallel) into row 2, this being done by the clock C₁ which is obtainedfrom the shaft encoder (FIG. 2). The next line of information is thenclocked in at the sample rate, C₃, and all previously stored lines ofinformation are then clocked in parallel to the next row using the clockC₁. This continues until a full field of information has been clockedinto the field store 210 and the first scanline of the field is in thelast row (row L), the second scanline of the field is in the next tolast row (row L-1) . . . and the last scanline of information is in thefirst row. Information is subsequently clocked out of the field store210 on a first-in-first-out basis. In particular, elemental samples areagain clocked out using the clock C₃, but now the lines of informationare clocked from row to row using the periodic clock C₂, which operatesat the line rate, as previously described. Information is clocked out ofrow L as indicated by the arrow. When a full line of information hasbeen clocked out using the sample clock C₃, the lines of information areclocked in parallel to the next row using the clock C₂, and the secondline of information is then clocked out using clock C₃, and so on. Thiscontinues until the entire field of information has been clocked out.The clocked out information is coupled via adder 290 which combines thesignals from the two field store units, and via filter 295, whichremoves the clock frequencies from the signal, to the display 11.

Operation of the field store 260 is similar to that of the field store210 except that the field store 260 clocks information in and out in aphase which is opposite to that of field store 210. Also, the fieldstore 260 is operative to clock lines of information out on alast-in-first-out basis. In other words, lines are clocked into the rowsof field store 260 in the same manner described with respect to fieldstore 210, but during the clocking out of lines the information in thevarious rows is shifted in the opposite direction from the directionduring clocking in, such that elemental samples are clocked out from theend of row 1 (rather than from the end of row L as is the case for fieldstore 210).

Overall operation of the frame storage subsystem 200 is as follows:During scanning by the reflector 70 in one direction, lines ofecho-representative information are clocked into the "odd" field store210 in the manner described. During scanning of the reflector in theopposite direction, the information in "odd" field store is clocked outfor display and the new echo-representative information is clocked intothe "even" field store 260. Subsequently, while the information nowstored in field store 260 is being clocked out for display, newinformation is being clocked into field store 210, and so on. Since theinformation from the two fields is combined to form a single frame, andthe beam is scanning in opposite directions the two fields, the fieldsare properly correlated by reading out the lines of field store 210 on afirst-in-first-out basis and the lines of field store 260 on alast-in-first-out basis. It should be noted that while the lineinformation during the two fields is clocked out in opposite senses, thesample information for each line may always be clocked out on alast-in-first-out basis.

Before referring to further circuits details of FIG. 4, it should beunderstood that the convention utilized in the present embodiment is asfollows: Recall that the two oppositely phased frame rate clocks aredesignated F and F. During each frame half-cycle (field) when F ispositive, the "odd" field store 210 shall be clocking information in andthe "even" field store 260 shall be clocking information out.Conversely, when the clock signal F is positive, the field store 260shall be clocking information in and the field store 210 shall beclocking information out.

Clocking in of samples to the field store 210 is under control of ANDgate 211 which couples the three-phase clock C₃ to the field store 210when enabled by positive cycles of the clock F. For clocking out of theelemental samples of each scanline, another AND gate 212 enables thethree phase clock C₃ during the positive portions of the clock signal F.The clocking into and out of the field store 260 is opposite in phasefrom the description of field store 210. In particular, the AND gate261, which controls clocking in, is enabled by the positive portions ofthe clock signal F and the clocking out is controlled by the AND gate262 which is enabled by the positive portions of the clock signal F. Theline clocking is achieved, with respect to field store 210, by theclocking in of line information with clock C₁ via AND gate 216, and thenclocking out of line information using the clock C₂ via the AND gate217. The AND gate 216 is enabled by the positive portions of the clocksignal F, and the AND gate 217 is enabled by the positive portions ofthe clock signal F. Accordingly, and consistent with the above-listedconvention, clocking in with the clock C₁ is performed every other fieldand clocking out using the clock C₂ is performed during the interveningfields. The opposite is true in the case of the field store 260 whereinthe AND gate 266, which receives the clock C₁ as one input, is enabledby the positive portions of clock F, and the AND gate 267, whichreceives the clock C₂ as one input, is enabled by the positive portionsof the clock signal F. In the case of field store 260, the row-to-rowconnections for shifting line information in the field store are inreverse relationship for clocking in and clocking out, this being doneso that the packets of charge are "moved" in opposite directions everyother field cycle, to achieve last-in-first-out operation for the linesof information.

To better understand operation of the invention, reference is made tothe timing diagrams of FIG. 5 which depict various signals in thepresent embodiment. FIG. 5A shows the sinusoidal drive which is outputof the oscillator 183 and operative to drive the reflector 70 inoscillatory motion. The frequency of the sinusoid corresponds to theframe rate, F. During the rise of the signal, the reflector 70 isscanned from left to right, causing the beam to also scan from left toright. During the fall time of the sinusoid, the reflector 70 is causedto scan back from right to left (the portion of the scan that isgenerally used for "flyback"). FIGS. 5B and 5C show the frame rate clocksignals F and F and FIG. 5D shows the field rate clock 2F from which thevertical sweep signal V for the display is derived by sweep circuit 182(FIG. 3). FIG. 5E illustrates the nature of the clock signal C₁ derivedfrom the shaft encoder 73 and FIG. 5F depicts the nature of the clocksignal C₂ generated as shown in FIG. 3. In FIGS. 5E and 5F the verticallines represent clock pulses and the actual number of such pulses is notintended to represent the actual number of lines during each field,which would typically be larger than that shown. It is seen that in thecase of the clock C₁, which is derived from the shaft encoder, thepulses are further apart at those positions near where the reflector isabout to change direction or has just changed direction (i.e., nearwhere the sinusoid has a slope close to zero) and are closer togetherwhen the reflector is travelling at maximum speed near the center ofeach excursion from right to left or left to right (i.e., where thesinusoid has maximum slope). This pattern results from obtaining theclock C₁ as a function of the relative angular position of the reflector70. The clock C₂ is periodic and is generated at the display line rateby the divider of FIG. 3. During the first half cycle of the frame rateclock, F, the clock pulses C₁ are used to clock lines of informationinto the field store 210 (by operation of gate 216 which is enabled byF) and the clock C₂ is used to clock lines of information out of fieldstore 260 (by operation of gate 267 which is also enabled by F). Duringthe next half cycle of the frame rate clock (during which F ispositive), the clock C₁ is utilized to clock the lines of the next fieldof information into the field store 260 (by virtue of gate 266 beingenabled by F) and the clock C₂ is utilized to clock lines of informationout of the field store 210 (by virtue of gate 217 being enabled by F).Accordingly, it is seen that the scanner 70 can be driven in anadvantageous manner from the standpoint of mechanical dynamics, and thatthe duty cycle of operation is enhanced. The number of apertures in theapertured wheel 73D of shaft encoder 73 should preferably be set equalto L so that the number of clock pulses C₁ during a field equals thenumber of clock pulses C₂.

I claim:
 1. Apparatus for scanning an object with a beam of ultrasoundenergy and for formulating an image from the ultrasound reflected fromthe object, comprising:an ultrasound reflector disposed in the path ofsaid ultrasound energy; a fluid surrounding said reflector; means formechanically driving said reflector in an oscillatory fashion at anon-linear rate; means for sensing the relative angular position of saidreflector; means for generating a first continuously variable clocksignal as a function of the sensed position; means responsive to theultrasound reflected from said object for generating echo-representativeelectrical signals; means for storing said electrical signals at a linerate which depends upon said first clock signal; means for generating asecond periodic clock signal; means for reading out the stored signalsat a line rate which depends upon said second clock signal; and meansfor displaying the read out signals to obtain an image of the object,the line rate of said display being synchronized with said second clocksignal.
 2. Apparatus as defined by claim 1 wherein said means fordriving the reflector is adapted to drive the reflector substantiallysinusoidally.
 3. Apparatus as defined by claim 1 wherein said means forsensing the position of said reflector comprises a shaft encoder. 4.Apparatus as defined by claim 1 wherein said means for storing theecho-representative electrical signals comprises a charge transferregister.
 5. Apparatus as defined by claim 1 wherein said means forstoring the echo-representative electrical signals comprises a pair ofcharge transfer registers.
 6. Apparatus as defined by claim 5 furthercomprising means for effecting storage of said echo-representativeelectrical signals in one of said charge transfer registers during onehalf-cycle of a reflector drive period and for effecting storage of saidecho-representative electrical signals in the other of said chargetransfer registers during the other half-cycle of a reflector driveperiod.
 7. Apparatus as defined by claim 6 wherein said means foreffecting storage of said echo-representative signals is also operativeto effect the reading out of lines of said stored signals at said secondclock rate from each of said charge transfer registers during the timethat the other charge transfer register is storing theecho-representative electrical signals at said first clock rate. 8.Apparatus as defined by claim 7 wherein one of said charge transferregisters is a field store operative to read out lines of information ona first-in-first-out basis and the other of said charge transferregisters is a field store operative to read out lines of information ona last-in-first-out basis.